In recent years, problems related to aging of structures such as bridges constructed during the period of high economic growth have become noticeable. Because loss is immeasurable when an accident occurs in a structure, technologies for monitoring a state of a structure have been proposed. For example, a technology for detecting damage to a structure by an acoustic emission (AE) method in which an elastic wave generated due to occurrence of an internal crack or progress of an internal crack is detected by a high-sensitivity sensor has been proposed. AE is an elastic wave generated due to a progress of fatigue crack of a material. In the AE method, an elastic wave is detected as an AE signal (voltage signal) by an AE sensor using a piezoelectric element. The AE signal is detected as an indication before breakage of the material occurs. Therefore, the frequency of occurrence of AE signals and the signal intensity are useful as an index indicating the soundness of the material. For this reason, studies are being carried out on technologies for detecting signs of deterioration of structures by the AE method.
A tomographic method is known as one of methods for evaluating a structure using an AE signal. Tomography is one of inverse analysis methods for estimating a velocity field structure inside a measurement region by using an arrival time difference between acoustic signals detected by a plurality of sensors, and can be used as a nondestructive inspection method for detecting a damaged part as a change in velocity field. Particularly, a method using an AE signal generated from inside a material as a signal source is known as AE tomography. However, in the conventional method, it is necessary to calculate component parameters iteratively by a simultaneous iterative method until a residual difference between a measured travel time and a theoretical travel time converges to within a tolerance range. For this reason, a considerable amount of calculation time is required for evaluating a structure in some cases.